Update asciidoctor-pdf requirement from 1.5.0.alpha.16 to 1.5.3 (#130)

Updates the requirements on [asciidoctor-pdf](https://github.com/asciidoctor/asciidoctor-pdf) to permit the latest version.
- [Release notes](https://github.com/asciidoctor/asciidoctor-pdf/releases)
- [Changelog](https://github.com/asciidoctor/asciidoctor-pdf/blob/master/CHANGELOG.adoc)
- [Commits](asciidoctor/asciidoctor-pdf@v1.5.0.alpha.16...v1.5.3)

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Gemfile

@@ -7,7 +7,7 @@ gem 'json'
 gem 'awesome_print'
 
 gem 'asciidoctor-epub3', :git => 'https://github.com/asciidoctor/asciidoctor-epub3'
-gem 'asciidoctor-pdf', '1.5.0.alpha.16'
+gem 'asciidoctor-pdf', '1.5.3'
 
 gem 'coderay'
 gem 'pygments.rb'